This invention generally relates to methods for designing serif masks for photolithography.
Photolithography is the technology of reproducing patterns using light. As presently used within semiconductor technology, a mask pattern for a desired circuit is transferred to a wafer through light exposure, development, etch, and resist strip, etc. As the feature size on a circuit becomes smaller and smaller, the circuit shape on the wafer differs from the original mask pattern more and more. This effect is due to local and far-range proximity effects, including diffraction, exposure, baking, dissolution and etching factors. In particular, corner rounding, line end foreshortening and width changes of lines are typically observed, resulting in, for example, nested lines printing differently from isolated ones.
A key reason for optical proximity effects is due to light diffraction. Optical proximity effects coming from light diffraction can be overcome partly if one has the choice of using a shorter wavelength source of light, with a projection system possessing a larger numerical aperture. In practice, the wavelength of an optical light source is typically fixed (365 nm, 248 nm, 193 nm, etc.) and there is a practical upper limit on numerical aperture. So other resolution enhancement methods, including the use of phase-shifting masks and masks with serifs, have been developed to correct optical proximity effects.